Photosynthesis — AQA GCSE Biology
The photosynthesis equation, the factors that limit its rate, the required practical with pondweed, and what plants do with the glucose they make.
Revision notes
5 short notes, in the order of the specification. Each one in short:
Photosynthesis is the reaction in which plants use carbon dioxide and water to make glucose and oxygen: carbon dioxide + water → glucose + oxygen. It happens in the chloroplasts, which contain chlorophyll. It is an endothermic reaction, because energy is transferred from the environment to the chloroplasts by light, so it needs a constant supply of light.
The rate of photosynthesis is affected by four factors: light intensity, carbon dioxide concentration, temperature and the amount of chlorophyll. The one in short supply is the limiting factor, and raising other factors makes no difference until it is increased. Above the optimum temperature, the enzymes denature. Rate = volume of oxygen produced ÷ time.
Light intensity, carbon dioxide concentration and temperature interact, and any one of them may limit the rate of photosynthesis. On a graph, the factor on the x-axis is limiting where a curve rises. Light intensity follows the inverse square law: it is proportional to 1 ÷ distance², so doubling the distance gives one quarter of the intensity.
This required practical investigates how light intensity affects the rate of photosynthesis using pondweed. A lamp is placed at different distances, and the oxygen bubbles released in a set time are counted, or the volume of gas is collected. Temperature and carbon dioxide are controlled. The closer the lamp, the higher the rate, until another factor becomes limiting.
The glucose made in photosynthesis is used for respiration, converted into insoluble starch for storage, used to make fat or oil for storage, used to make cellulose to strengthen cell walls, and used to make amino acids for protein synthesis. To make amino acids, plants also need nitrate ions absorbed from the soil as a source of nitrogen.
Exam questions
11 exam-style questions (35 marks), each with its mark scheme.
Answer the questionsFlashcards
25 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of bioenergetics on one page, so you can see where this subtopic fits.
Open the mind mapPractical sheet
Method, variables, risks and a results table, with questions to answer.
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Quick check questions
Write the word equation for photosynthesis.
Show answer
carbon dioxide + water → glucose + oxygen (in the presence of light)
Name the four factors that affect the rate of photosynthesis.
Show answer
Temperature, light intensity, carbon dioxide concentration and the amount of chlorophyll.
In the pondweed practical, what is the independent variable?
Show answer
The distance of the lamp from the pondweed (which changes the light intensity).
Why do plants need nitrate ions?
Show answer
To make proteins, as nitrate ions are used with glucose to make amino acids.
Why is photosynthesis described as endothermic?
Show answer
It takes in energy from the environment, transferred to the chloroplasts by light.
Frequently asked questions
What are the limiting factors of photosynthesis?
The limiting factors of photosynthesis are light intensity, carbon dioxide concentration, temperature and the amount of chlorophyll. A limiting factor is the one in short supply, so it limits the rate. Increasing a different factor makes no difference until the limiting factor is increased. Which factor is limiting can change as conditions change.
How does light intensity affect the rate of photosynthesis?
Increasing light intensity increases the rate of photosynthesis, because light provides the energy for the reaction. The rate rises up to a point, then levels off when light is no longer the limiting factor and another factor, such as carbon dioxide concentration or temperature, is in short supply. In the pondweed practical, a closer lamp gives more bubbles.
How does temperature affect the rate of photosynthesis?
The rate of photosynthesis increases as temperature increases, up to an optimum. Photosynthesis is controlled by enzymes, and warmer particles move faster and collide more often. Above the optimum temperature the enzymes denature, so the rate drops quickly. In the pondweed practical, the water temperature is kept steady so that it does not affect the results.
What are the products of photosynthesis used for?
Photosynthesis produces glucose and oxygen. Plants use the glucose for respiration, convert it into insoluble starch for storage, use it to make fat or oil for storage, make cellulose to strengthen cell walls, and make amino acids for protein synthesis, using nitrate ions from the soil. The oxygen is released, which is why pondweed gives off bubbles in light.
What is the inverse square law in photosynthesis?
Higher tier The inverse square law states that light intensity is inversely proportional to the square of the distance from the light source. If a lamp is moved twice as far from a plant, the light intensity falls to one quarter, not one half. When light is the limiting factor, the rate of photosynthesis is roughly proportional to light intensity.
Written and checked against the AQA GCSE Biology (8461) specification · Updated October 2026